Throughout the world, there are many different types of industrial water systems. Industrial water systems exist so that necessary chemical, mechanical and biological processes can be conducted to reach the desired outcome. Fouling can occur even in industrial water systems treated with the best water treatment programs currently available. For purposes of this patent application “fouling” is defined as “the deposition of any organic or inorganic material on a surface”.
If these industrial water systems are not periodically cleaned, then they will become heavily fouled. Fouling has a negative impact on the industrial water system. For example, severe mineral scale (inorganic material) will buildup on the water contact surfaces and anywhere there is scale, there is an ideal environment for the growth of microorganisms.
Evaporative cooling water systems are particularly prone to fouling. This fouling occurs by a variety of mechanisms including deposition of air-borne and water-borne and water-formed contaminants, water stagnation, process leaks, and other factors. If allowed to progress, the system can suffer from decreased operational efficiency, premature equipment failure, and increased health-related risks associated with microbial fouling.
Fouling can also occur due to microbiological contamination. Sources of microbial contamination in industrial water systems are numerous and may include, but are not limited to, air-borne contamination, water make-up, process leaks and improperly cleaned equipment. These microorganisms can establish microbial communities on any wetable or semi-wetable surface of the water system. Once these microbial populations are present in the bulk water more than 99% of the microbes present in the water will be present on all surfaces.
Exopolymeric substance secreted from the microorganisms aid in the formation of biofilms as the microbial communities develop on the surface. These biofilms are complex ecosystems that establish a means for concentrating nutrients and offer protection for growth, and biofilms can accelerate scale, corrosion, and other fouling processes. Not only do biofilms contribute to reduction of system efficiencies, but they also provide an excellent environment for microbial proliferation that can include legionellae bacteria. It is therefore important that biofilms and other fouling processes be reduced to the greatest extent possible to minimize the health-related risk associated with legionellae and other water-borne pathogens.
There are several different cleaning or disinfection methods for cooling water systems. For example, mechanical cleaning, hyperhalogenation with and without surfactants or dispersants, and acid cleaning, are amongst the cleaning methods most commonly used.
A simple mechanical cleaning program consists of “power washing” and “scrubbing”. Power washing refers to the use of high-pressure water directed at equipment surfaces such that the impact of the water on the surface removes deposits from those surfaces that can be reached. Mechanical cleaning strategies do not always remove all heavily adhering deposits such as deposited scale and biological slime from equipment surfaces. Further limitations on the use of mechanical cleaning include the fact that such methods are effective at removing only loose deposits, and not for removing deposits from within the fill (in the case of a cooling tower). For systems such as domestic water distribution pipes, power washing is completely ineffective. Mechanical cleaning methods also do not provide a means of disinfection, which is crucial to maintaining clean and safe equipment.
One standard procedure that uses hyperhalogenation and surfactant is commonly known as the “Wisconsin Protocol”, see “CONTROL OF LEGIONELLA IN COOLING TOWERS”, Summary Guidelines, Section of Acute and Communicable Disease Epidemiology, Bureau of Community Health and Prevention, Division of Health, Wisconsin Department of Health and Social Services, August 1987. The Wisconsin Protocol is used to disinfect an industrial water system following a legionellosis outbreak. Even in the absence of an outbreak, hyperhalogenation is commonly used to reduce microbial fouling in water systems. Hyperhalogenation protocols do not remove mineral scale, thereby limiting their ability to remove or reduce biological foulants. In addition, the hyperhalogenation procedures that require a biodispersant have limited effectiveness within the period the protocol is implemented. The use of certain acids, such as sulfuric acid, in combination with high halogen doses, as specified in the Wisconsin Protocol, can form copper sulfate and other deposits that are subsequently difficult to remove. Finally, because the hyperhalogenation methods do not remove scale and other deposits sufficiently, microbial populations re-establish rapidly in the systems.
An “acid cleaning procedure” is designed to remove mineral scale. Acid is capable of removing alkaline scale from virtually all wetable surfaces. Acid cleaning procedures offer limited disinfection because of the lowered pH, but do not adequately penetrate biological deposits (biofilms) that remain associated with system surfaces.
Operators of industrial water systems use chlorine dioxide to kill microorganisms. Chlorine dioxide is a well-known biocide, but does not have the ability to remove mineral scale. Chlorine dioxide must be generated on-site where it is applied. There are several methods for generating and delivering chlorine dioxide. One of these methods uses acid in combination with sodium chlorite (acid activation). For example, the chlorine dioxide is generated using sodium chlorite and hydrochloric acid as follows:5NaClO2+4HCl=4ClO2+5NaCl+2H2OTypically, the reactants (sodium chlorite and hydrochloric acid) are mixed and allowed to react to form chlorine dioxide. Following this reaction, the products (chlorine dioxide, sodium chloride, water, some remaining unreacted sodium chlorite, and hydrochloric acid) are added directly into the water of the industrial water system. Once this externally-generated chlorine dioxide solution is applied to the water system, the chlorine dioxide is diluted and either lost through volatility or is reduced by substances within the water system. Using this method, chlorine dioxide must be constantly generated outside the water system, and injected into the system to maintain a chlorine dioxide residual.
Patent Cooperation Treaty Patent Application WO 02/12130 A1 describes and claims a method of treating water in a water distribution system, comprising:
admixing a sodium chlorite solution with a second solution containing an acid to make a reacted mixture; and
introducing a predetermined amount of the reacted mixture into a water system.
As described in this Patent Cooperation Treaty Patent Application, the preferred method of treating water includes the addition of a catalyst, wherein the catalyst is sodium molybdate.
United Kingdom Patent Application No. 2,313,369 describes and claims an aqueous composition having a pH of more than 9 consists of a stabilized chlorine dioxide precursor, an alkali metal polyphosphate and an alkali metal hydroxide. It also describes and claims a method of treating water in a water distribution system comprising the addition of an acid activator to the aqueous composition to reduce the pH to less than 7 and injecting the aqueous solution in to the water system.
It would be desirable to have a method for simultaneously cleaning and disinfecting an industrial water system.